pyosys: Use C++11 override keyword for bindings

pyosys: Use C++11 override keyword for bindings

7191dd16 dropped the YS_OVERRIDE macro, but it was still being generated
by the python bindings generator, resulting in errors like these when
compiled with ENABLE_PYOSYS=1:

kernel/python_wrappers.cc:350:21: error: expected ‘;’ at end of member declaration
  350 |   virtual void help() YS_OVERRIDE;
      |                     ^
      |                      ;
kernel/python_wrappers.cc:350:23: error: ‘YS_OVERRIDE’ does not name a type
  350 |   virtual void help() YS_OVERRIDE;
      |                       ^~~~~~~~~~~